com.unity.ml-agents.extensions/Runtime/Sensors/PoseExtractor.cs

using System;
using System.Collections.Generic;
using UnityEngine;
using Object = UnityEngine.Object;

namespace Unity.MLAgents.Extensions.Sensors
{
    /// <summary>
    /// Abstract class for managing the transforms of a hierarchy of objects.
    /// This could be GameObjects or Monobehaviours in the scene graph, but this is
    /// not a requirement; for example, the objects could be rigid bodies whose hierarchy
    /// is defined by Joint configurations.
    ///
    /// Poses are either considered in model space, which is relative to a root body,
    /// or in local space, which is relative to their parent.
    /// </summary>
    public abstract class PoseExtractor
    {
        int[] m_ParentIndices;
        Pose[] m_ModelSpacePoses;
        Pose[] m_LocalSpacePoses;

        Vector3[] m_ModelSpaceLinearVelocities;
        Vector3[] m_LocalSpaceLinearVelocities;

        bool[] m_PoseEnabled;


        /// <summary>
        /// Read iterator for the enabled model space transforms.
        /// </summary>
        public IEnumerable<Pose> GetEnabledModelSpacePoses()
        {
            if (m_ModelSpacePoses == null)
            {
                yield break;
            }

            for (var i = 0; i < m_ModelSpacePoses.Length; i++)
            {
                if (m_PoseEnabled[i])
                {
                    yield return m_ModelSpacePoses[i];
                }
            }
        }

        /// <summary>
        /// Read iterator for the enabled local space transforms.
        /// </summary>
        public IEnumerable<Pose> GetEnabledLocalSpacePoses()
        {
            if (m_LocalSpacePoses == null)
            {
                yield break;
            }

            for (var i = 0; i < m_LocalSpacePoses.Length; i++)
            {
                if (m_PoseEnabled[i])
                {
                    yield return m_LocalSpacePoses[i];
                }
            }
        }

        /// <summary>
        /// Read iterator for the enabled model space linear velocities.
        /// </summary>
        public IEnumerable<Vector3> GetEnabledModelSpaceVelocities()
        {
            if (m_ModelSpaceLinearVelocities == null)
            {
                yield break;
            }

            for (var i = 0; i < m_ModelSpaceLinearVelocities.Length; i++)
            {
                if (m_PoseEnabled[i])
                {
                    yield return m_ModelSpaceLinearVelocities[i];
                }
            }
        }

        /// <summary>
        /// Read iterator for the enabled local space linear velocities.
        /// </summary>
        public IEnumerable<Vector3> GetEnabledLocalSpaceVelocities()
        {
            if (m_LocalSpaceLinearVelocities == null)
            {
                yield break;
            }

            for (var i = 0; i < m_LocalSpaceLinearVelocities.Length; i++)
            {
                if (m_PoseEnabled[i])
                {
                    yield return m_LocalSpaceLinearVelocities[i];
                }
            }
        }

        /// <summary>
        /// Number of enabled poses in the hierarchy (read-only).
        /// </summary>
        public int NumEnabledPoses
        {
            get
            {
                if (m_PoseEnabled == null)
                {
                    return 0;
                }

                var numEnabled = 0;
                for (var i = 0; i < m_PoseEnabled.Length; i++)
                {
                    numEnabled += m_PoseEnabled[i] ? 1 : 0;
                }

                return numEnabled;
            }
        }

        /// <summary>
        /// Number of total poses in the hierarchy (read-only).
        /// </summary>
        public int NumPoses
        {
            get { return m_ModelSpacePoses?.Length ?? 0; }
        }

        /// <summary>
        /// Get the parent index of the body at the specified index.
        /// </summary>
        /// <param name="index"></param>
        /// <returns></returns>
        public int GetParentIndex(int index)
        {
            if (m_ParentIndices == null)
            {
                throw new NullReferenceException("No parent indices set");
            }

            return m_ParentIndices[index];
        }

        /// <summary>
        /// Set whether the pose at the given index is enabled or disabled for observations.
        /// </summary>
        /// <param name="index"></param>
        /// <param name="val"></param>
        public void SetPoseEnabled(int index, bool val)
        {
            m_PoseEnabled[index] = val;
        }

        public bool IsPoseEnabled(int index)
        {
            return m_PoseEnabled[index];
        }

        /// <summary>
        /// Initialize with the mapping of parent indices.
        /// The 0th element is assumed to be -1, indicating that it's the root.
        /// </summary>
        /// <param name="parentIndices"></param>
        protected void Setup(int[] parentIndices)
        {
#if DEBUG
            if (parentIndices[0] != -1)
            {
                throw new UnityAgentsException($"Expected parentIndices[0] to be -1, got {parentIndices[0]}");
            }
#endif
            m_ParentIndices = parentIndices;
            var numPoses = parentIndices.Length;
            m_ModelSpacePoses = new Pose[numPoses];
            m_LocalSpacePoses = new Pose[numPoses];

            m_ModelSpaceLinearVelocities = new Vector3[numPoses];
            m_LocalSpaceLinearVelocities = new Vector3[numPoses];

            m_PoseEnabled = new bool[numPoses];
            // All poses are enabled by default. Generally we'll want to disable the root though.
            for (var i = 0; i < numPoses; i++)
            {
                m_PoseEnabled[i] = true;
            }
        }

        /// <summary>
        /// Return the world space Pose of the i'th object.
        /// </summary>
        /// <param name="index"></param>
        /// <returns></returns>
        protected internal abstract Pose GetPoseAt(int index);

        /// <summary>
        /// Return the world space linear velocity of the i'th object.
        /// </summary>
        /// <param name="index"></param>
        /// <returns></returns>
        protected internal abstract Vector3 GetLinearVelocityAt(int index);

        /// <summary>
        /// Return the underlying object at the given index. This is only
        /// used for display in the inspector.
        /// </summary>
        /// <param name="index"></param>
        /// <returns></returns>
        protected internal virtual Object GetObjectAt(int index)
        {
            return null;
        }

        /// <summary>
        /// Update the internal model space transform storage based on the underlying system.
        /// </summary>
        public void UpdateModelSpacePoses()
        {
            using (TimerStack.Instance.Scoped("UpdateModelSpacePoses"))
            {
                if (m_ModelSpacePoses == null)
                {
                    return;
                }

                var rootWorldTransform = GetPoseAt(0);
                var worldToModel = rootWorldTransform.Inverse();
                var rootLinearVel = GetLinearVelocityAt(0);

                for (var i = 0; i < m_ModelSpacePoses.Length; i++)
                {
                    var currentWorldSpacePose = GetPoseAt(i);
                    var currentModelSpacePose = worldToModel.Multiply(currentWorldSpacePose);
                    m_ModelSpacePoses[i] = currentModelSpacePose;

                    var currentBodyLinearVel = GetLinearVelocityAt(i);
                    var relativeVelocity = currentBodyLinearVel - rootLinearVel;
                    m_ModelSpaceLinearVelocities[i] = worldToModel.rotation * relativeVelocity;
                }
            }
        }

        /// <summary>
        /// Update the internal model space transform storage based on the underlying system.
        /// </summary>
        public void UpdateLocalSpacePoses()
        {
            using (TimerStack.Instance.Scoped("UpdateLocalSpacePoses"))
            {
                if (m_LocalSpacePoses == null)
                {
                    return;
                }

                for (var i = 0; i < m_LocalSpacePoses.Length; i++)
                {
                    if (m_ParentIndices[i] != -1)
                    {
                        var parentTransform = GetPoseAt(m_ParentIndices[i]);
                        // This is slightly inefficient, since for a body with multiple children, we'll end up inverting
                        // the transform multiple times. Might be able to trade space for perf here.
                        var invParent = parentTransform.Inverse();
                        var currentTransform = GetPoseAt(i);
                        m_LocalSpacePoses[i] = invParent.Multiply(currentTransform);

                        var parentLinearVel = GetLinearVelocityAt(m_ParentIndices[i]);
                        var currentLinearVel = GetLinearVelocityAt(i);
                        m_LocalSpaceLinearVelocities[i] = invParent.rotation * (currentLinearVel - parentLinearVel);
                    }
                    else
                    {
                        m_LocalSpacePoses[i] = Pose.identity;
                        m_LocalSpaceLinearVelocities[i] = Vector3.zero;
                    }
                }
            }
        }

        /// <summary>
        /// Compute the number of floats needed to represent the poses for the given PhysicsSensorSettings.
        /// </summary>
        /// <param name="settings"></param>
        /// <returns></returns>
        public int GetNumPoseObservations(PhysicsSensorSettings settings)
        {
            int obsPerPose = 0;
            obsPerPose += settings.UseModelSpaceTranslations ? 3 : 0;
            obsPerPose += settings.UseModelSpaceRotations ? 4 : 0;
            obsPerPose += settings.UseLocalSpaceTranslations ? 3 : 0;
            obsPerPose += settings.UseLocalSpaceRotations ? 4 : 0;

            obsPerPose += settings.UseModelSpaceLinearVelocity ? 3 : 0;
            obsPerPose += settings.UseLocalSpaceLinearVelocity ? 3 : 0;

            return NumEnabledPoses * obsPerPose;
        }

        internal void DrawModelSpace(Vector3 offset)
        {
            UpdateLocalSpacePoses();
            UpdateModelSpacePoses();

            var pose = m_ModelSpacePoses;
            var localPose = m_LocalSpacePoses;
            for (var i = 0; i < pose.Length; i++)
            {
                var current = pose[i];
                if (m_ParentIndices[i] == -1)
                {
                    continue;
                }

                var parent = pose[m_ParentIndices[i]];
                Debug.DrawLine(current.position + offset, parent.position + offset, Color.cyan);
                var localUp = localPose[i].rotation * Vector3.up;
                var localFwd = localPose[i].rotation * Vector3.forward;
                var localRight = localPose[i].rotation * Vector3.right;
                Debug.DrawLine(current.position + offset, current.position + offset + .1f * localUp, Color.red);
                Debug.DrawLine(current.position + offset, current.position + offset + .1f * localFwd, Color.green);
                Debug.DrawLine(current.position + offset, current.position + offset + .1f * localRight, Color.blue);
            }
        }

        /// <summary>
        /// Simplified representation of the a node in the hierarchy for display.
        /// </summary>
        internal struct DisplayNode
        {
            /// <summary>
            /// Underlying object in the hierarchy. Pass to EditorGUIUtility.ObjectContent() for display.
            /// </summary>
            public Object NodeObject;

            /// <summary>
            /// Whether the poses for the object are enabled.
            /// </summary>
            public bool Enabled;

            /// <summary>
            /// Depth in the hierarchy, used for adjusting the indent level.
            /// </summary>
            public int Depth;

            /// <summary>
            /// The index of the corresponding object in the PoseExtractor.
            /// </summary>
            public int OriginalIndex;
        }

        /// <summary>
        /// Get a list of display nodes in depth-first order.
        /// </summary>
        /// <returns></returns>
        internal IList<DisplayNode> GetDisplayNodes()
        {
            if (NumPoses == 0)
            {
                return Array.Empty<DisplayNode>();
            }
            var nodesOut = new List<DisplayNode>(NumPoses);

            // List of children for each node
            var tree = new Dictionary<int, List<int>>();
            for (var i = 0; i < NumPoses; i++)
            {
                var parent = GetParentIndex(i);
                if (i == -1)
                {
                    continue;
                }

                if (!tree.ContainsKey(parent))
                {
                    tree[parent] = new List<int>();
                }
                tree[parent].Add(i);
            }

            // Store (index, depth) in the stack
            var stack = new Stack<(int, int)>();
            stack.Push((0, 0));

            while (stack.Count != 0)
            {
                var (current, depth) = stack.Pop();
                var obj = GetObjectAt(current);

                var node = new DisplayNode
                {
                    NodeObject = obj,
                    Enabled = IsPoseEnabled(current),
                    OriginalIndex = current,
                    Depth = depth
                };
                nodesOut.Add(node);

                // Add children
                if (tree.ContainsKey(current))
                {
                    // Push to the stack in reverse order
                    var children = tree[current];
                    for (var childIdx = children.Count - 1; childIdx >= 0; childIdx--)
                    {
                        stack.Push((children[childIdx], depth + 1));
                    }
                }

                // Safety check
                // This shouldn't even happen, but in case we have a cycle in the graph
                // exit instead of looping forever and eating up all the memory.
                if (nodesOut.Count > NumPoses)
                {
                    return nodesOut;
                }
            }

            return nodesOut;
        }
    }

    /// <summary>
    /// Extension methods for the Pose struct, in order to improve the readability of some math.
    /// </summary>
    public static class PoseExtensions
    {
        /// <summary>
        /// Compute the inverse of a Pose. For any Pose P,
        ///   P.Inverse() * P
        /// will equal the identity pose (within tolerance).
        /// </summary>
        /// <param name="pose"></param>
        /// <returns></returns>
        public static Pose Inverse(this Pose pose)
        {
            var rotationInverse = Quaternion.Inverse(pose.rotation);
            var translationInverse = -(rotationInverse * pose.position);
            return new Pose { rotation = rotationInverse, position = translationInverse };
        }

        /// <summary>
        /// This is equivalent to Pose.GetTransformedBy(), but keeps the order more intuitive.
        /// </summary>
        /// <param name="pose"></param>
        /// <param name="rhs"></param>
        /// <returns></returns>
        public static Pose Multiply(this Pose pose, Pose rhs)
        {
            return rhs.GetTransformedBy(pose);
        }

        /// <summary>
        /// Transform the vector by the pose. Conceptually this is equivalent to treating the Pose
        /// as a 4x4 matrix and multiplying the augmented vector.
        /// See https://en.wikipedia.org/wiki/Affine_transformation#Augmented_matrix for more details.
        /// </summary>
        /// <param name="pose"></param>
        /// <param name="rhs"></param>
        /// <returns></returns>
        public static Vector3 Multiply(this Pose pose, Vector3 rhs)
        {
            return pose.rotation * rhs + pose.position;
        }

        // TODO optimize inv(A)*B?
    }
}